In this disclosure, the term “wireless device” is used to represent any communication entity capable of radio communication with a wireless network by sending and receiving radio signals, such as e.g. mobile telephones, tablets, laptop computers and Machine-to-Machine, M2M, devices, also known as Machine Type Communication, MTC, devices. Another common generic term in this field is “User Equipment, UE” which is frequently used herein as a synonym for wireless device.
Further, the term “network node”, is used herein to represent any node of a wireless network that is operative to communicate radio signals with wireless devices, or to control some network entity having radio equipment for receiving/transmitting the radio signals. The network node in this disclosure could also be referred to as a base station, radio node, e-NodeB, eNB, NB, base transceiver station, access point, etc., depending on the type of wireless network and terminology used. The term eNB is frequently used herein as a synonym for network node.
Location-based services and emergency call positioning are areas where positioning of wireless devices in wireless networks is needed, and a plethora of applications and services in the wireless devices take advantage of the position. Positioning in wireless networks employing Long Term Evolution, LTE, is supported by an architecture illustrated in FIG. 1a, employing direct interactions between a wireless device, here denoted UE, and a location server called Evolved-Serving Mobile Location Centre, E-SMLC, via the LTE Positioning Protocol, LPP. Moreover, there are also interactions between the location server and a network node denoted eNodeB, via the LPPa protocol, to some extent supported by interactions between the eNodeB and the UE via the Radio Resource Control, RRC, protocol.
The following positioning techniques are considered for use in LTE (3GPP 36.305):                Enhanced Cell ID. Essentially cell ID information to associate the UE to the serving area of a serving cell, and then additional information to determine the UE's position with greater accuracy.        Assisted GNSS. GNSS information retrieved by the UE, supported by assistance information provided to the UE from E-SMLC.        OTDOA (Observed Time Difference of Arrival). The UE estimates the time difference of reference signals from different base stations and sends the estimated time difference to the E-SMLC for multilateration.        UTDOA (Uplink TDOA). The UE is requested to transmit a specific waveform that is detected by multiple location measurement units such as base stations or eNBs located at known positions. These measurements are forwarded to the E-SMLC for multilateration.        
In the above positioning methods, it is necessary to estimate the time-of-arrival, TOA, of a radio signal received at a receiving node which signal has been transmitted from a transmitting node. The TOA information can be used to determine the distance between the transmitting node and the receiving node, assuming that the signal travels in air with the speed of light.
In a general scenario illustrated by FIG. 1b, a UE A is served/controlled by a serving/controlling node 100. In addition the UE A can possibly detect signals from one or more non-serving/non-controlling nodes 102. The UE A estimates the TOA of a signal from one transmitting node 100 or 102, possibly subject to an interfering signal from another node within coverage or communication range. Alternatively, significant interfering signals from other nodes may be avoided by employing a muting scheme where nodes alternating transmission and muting according to a schedule.
The above-described scenario may also be reversed, where TOA is estimated in a network node 100 or 102, based on a signal from a UE A, possibly subject to interference from a signal from a different UE B. Moreover, another scenario may also be that a UE A estimates a TOA based on a signal transmitted from a different UE, or a network node estimating TOA based on a signal transmitted from a different network node.
For example, the Observed Time Difference Of Arrival, OTDOA, is a UE-assisted method, in which the UE measures the TOA of specific Positioning Reference Signals, PRS, from multiple cells or eNBs, and computes the relative differences of TOAs between each cell and a reference cell. These Reference Signal Time Differences, RSTDs, are quantized and reported via LPP to the E-SMLC together with an accuracy assessment. Based on known positions of eNBs and their mutual time synchronization, it is possible for the E-SMLC to estimate the UE position from the RSTD and covariance reports using multilateration. The accuracy of this estimation depends on the radio conditions of the received signals, the number of received signals as well as the deployment, which means that it will vary spatially. FIG. 2 illustrates an example of multilateration in OTDOA based on signals transmitted from different network nodes eNB1-eNB3 while considering eNB1 as the reference cell.
One of the factors which significantly impacts on the performance of OTDOA, is the accuracy in the UE of estimating the TOA.